Surface treatment such as surface hardening by heat treatment, nitriding, etc., or shot-peening or the like are applied to steel parts such as gears and shafts used as automobile parts or the like to improve friction resistance, fatigue strength, or the like.
Conventionally, evaluation of surface properties such as residual stress and hardness following surface treatment of such products has been performed by sample destructive inspection. This led to the problem that not all products could be directly inspected, and inspected products became unusable, since inspection was destructive.
This has led to an increased need to develop a device capable of non-destructively inspecting product surface properties. As an example of such a device, Patent Citation 1 discloses a non-destructive inspection device for a shot-peened surface, wherein an AC (alternating current) signal is input while varying the frequency to an inspection circuit comprising a coil disposed above a shot-peened surface, and the state of occurrence of residual stress in the inspected object is inspected using impedance frequency response characteristics of the inspection circuit.